1. Field of Invention
The present invention relates to a substrate for an electronic device, a method for manufacturing this substrate for an electronic device, and an electronic device.
2. Description of the Related Art
In the related art, ferroelectric memories which are nonvolatile memories using ferroelectrics has been used.
These ferroelectric memories are classified into a capacitor type, in which a 1T/1C structure, etc., is formed using ferroelectrics as capacitors, and an MFSFET type, in which ferroelectrics are used as gate insulating films of field-effect transistors in place of SiO2.
The MFSFET type ferroelectric memory has advantages over the capacitor type, for example, it increases the packing density and non-destructive read-out. However, the MFSFET type is difficult to manufacture with respect to the structure. Thus, under present circumstances, development and commercialization of the capacitor type ferroelectric memory have been more advantageous.
Ferroelectric materials adopted in the capacitor type ferroelectric memories are divided into two materials, Pb(Zr1-xTix)O3 (PZT) and SrBi2Ta2O9 (SBT). Among them, the PZT materials having compositions in the neighborhood of the rhombohedron-tetragonal phase boundary (MPB) are superior in the residual dielectric polarization and coercive electric field property, and arc the materials which are more advantageous than the others in commercialization.
In the related art, Pt has been used as a PZT-based ferroelectric material for a lower electrode. Since Pt has a face-centered cubic lattice structure that is a close-packed structure, it has a strong self-orientation property, and therefore brings about cubic crystal orientation even on amorphous, such as SiO2.
However, since the orientation property is strong, there have been problems in that when a columnar crystal of Pt has grown, Pb, for example, is likely to diffuse in a substrate along grain boundaries, adhesion between Pt and SiO2 is degraded, and the like.
Although Ti may be used to enhance this adhesion between Pt and SiO2, and furthermore, TiN, etc., may be used as diffusion barrier layers against Pb, etc., electrode structures become complicated, and in addition, oxidation of Ti, diffusion of Ti into Pt, and reduction in crystallinity of PZT accompanying that are brought about, and therefore degradation of the polarization electric field (P-E) hysteresis characteristic, the leakage current characteristic, and the fatigue characteristic occur.
In order to address or avoid problems of Pt electrodes as described above, RuOx, IrO2, and other conductive oxide electrode materials have been researched. Among them, in particular, SrRuO3 having a perovskite structure has the same crystal structure as that of PZT, and therefore has superior joining property at the interface, is likely to realize epitaxial growth of PZT, and has superior characteristics as a diffusion barrier layer against Pb.
Consequently, the related art has researched ferroelectric capacitors using SrRuO3 as an electrode.
However, when the ferroelectric capacitor is configured using a metal oxide, for example, SrRuO3, having a perovskite structure as a lower electrode and using PZT as a ferroelectric, there have been problems as described below.
Regarding PZT, the composition of for example, Zr:Ti=0.3:0.7, which is on the excess Ti side compared with Zr:Ti=0.48:0.52 of MPB, is important from the viewpoint of an increase in residual dielectric polarization Pr and a decrease in coercive electric field Ec. In this composition range, PZT exhibits a tetragonal crystal, and the polarization direction thereof is parallel to the c axis.
Consequently, regarding the ferroelectric capacitor having a structure of upper electrode/ferroelectric layer/lower electrode/substrate, in order to produce a (001) orientation film of the ferroelectric layer PZT, it is necessary to make a SrRuO3 electrode as a lower electrode itself bring about pseudo-cubic crystal (100) orientation.
However, when the SrRuO3 electrode, which is a perovskite type metal oxide, is deposited directly on Si (substrate), since a SiO2 layer is formed at the interface, it is difficult to epitaxially grow SrRuO3.
Accordingly, it is necessary to epitaxially grow some buffer layer on Si (substrate), and to epitaxially grow a SrRuO3 electrode thereon.
Herein, examples of buffer layers which grow epitaxially on Si (substrate) with ease include metal oxides having a fluorite structure, for example, yttria-stabilized zirconia Zr1-xYxO2-0.5x (YSZ), CeO2, and Y2O3.
For example, it has been reported that a double buffer layers of Y2O3/YSZ (Appl. Phys. Lett., vol. 61 (1992) 1240) or CeO2/YSZ (Appl. Phys. Lett., vol. 64 (1994) 1573) is suitable as the buffer layer to grow epitaxially YBa2Cu3Ox having a structure similar to the perovskite. In this case, YBa2Cu3Ox brings about (001) orientation with ease.
However, (Appl. Phys. Lett., 67 (1995) 1387) discloses that SrRuO3 (in an orthorhombic crystal, a=0.5567 nm, b=0.5530 nm, and c=0.7845 nm, and in pseudo-cubic crystal, a=0.3923 nm and 21/2a=0.5548 nm) having a simple perovskite structure does not grow epitaxially with (100) orientation on a (100) plane of YSZ (a=0.514 nm) or CeO2 (a=0.541 nm), which has a fluorite structure, but brings about (110) orientation (pseudo-cubic crystal).
Accordingly, the inventor of the present invention researched materials for buffer layers which had a structure other than the fluorite structure and which grew epitaxially on Si (substrate) with ease. As a result, it was discovered that metal oxides having a NaCl structure were suitable, and therefore the present invention was made.
The present invention provides a substrate for an electronic device including a conductive oxide layer which is formed by epitaxial growth with cubic crystal (100) orientation or pseudo-cubic crystal (100) orientation and which contains a metal oxide having a perovskite structure, a method for manufacturing such a substrate for an electronic device, and an electronic device provided with such a substrate for an electronic device.
The above can be addressed or achieved by the present invention as described in the following (1) to (17).
(1) A substrate for an electronic device includes:
a Si substrate;
a buffer layer which is formed by epitaxial growth on the Si substrate and which contains a metal oxide having a NaCl structure; and
a conductive oxide layer which is formed by epitaxial growth with cubic crystal (100) orientation or pseudo-cubic crystal (100) orientation on the buffer layer and which contains a metal oxide having a perovskite structure.
(2) The substrate for an electronic device according to the aforementioned (1), where the Si substrate is a (100) substrate or a (110) substrate from which a natural oxidation film is not removed.
(3) The substrate for an electronic device according to the aforementioned (1) or (2), where the metal oxide having a NaCl structure is at least one of MgO, CaO, SrO, BaO, and solid solutions containing them.
(4) The substrate for an electronic device according to the aforementioned (1) or (2), where the aforementioned buffer layer has been grown epitaxially with cubic crystal (110) orientation.
(5) The substrate for an electronic device according to any one of the aforementioned (1) to (4), where the buffer layer has an average thickness of 10 nm or less.
(6) The substrate for an electronic device according to any one of the aforementioned (1) to (5), where the metal oxide having a perovskite structure is at least one of CaRuO3, SrRuO3, BaRuO3, and solid solutions containing them.
(7) A method for manufacturing a substrate for an electronic device, including:
cleaning a Si substrate;
forming a buffer layer containing a metal oxide having a NaCl structure by epitaxial growth, in which after the Si substrate is cleaned, the Si substrate in a vacuum apparatus is irradiated with plasma containing oxygen plasma and metal element plasma; and
forming a conductive oxide layer containing a metal oxide having a perovskite structure by epitaxial growth with cubic crystal (100) orientation or pseudo-cubic crystal (100) orientation, in which after the buffer layer is formed, the buffer layer in a vacuum apparatus is irradiated with plasma containing oxygen plasma and metal element plasma.
(8) The method for manufacturing a substrate for an electronic device according to the aforementioned (7), where in the cleaning the Si substrate, a treatment to produce a reconstructed surface or a hydrogen-terminated surface is not performed.
(9) The method for manufacturing a substrate for an electronic device according to aforementioned (8), where in the forming the buffer layer, the buffer layer is grown epitaxially while a natural oxidation film on the Si substrate surface is removed by irradiating the Si substrate selectively with the metal element plasma rather than the oxygen plasma.
(10) The method for manufacturing a substrate for an electronic device according to any one of the aforementioned (7) to (9), where the metal oxide having a NaCl structure is at least one of MgO, CaO, SrO, BaO, and solid solutions containing them.
(11) The method for manufacturing a substrate for an electronic device according to any one of the aforementioned (7) to (10), where the buffer layer is grown epitaxially with cubic crystal (110) orientation.
(12) The method for manufacturing a substrate for an electronic device according to any one of the aforementioned (7) to (11), where the buffer layer has an average thickness of 10 nm or less.
(13) The method for manufacturing a substrate for an electronic device according to any one of the aforementioned (7) to (12), where the metal oxide having a perovskite structure is at least one of CaRuO3, SrRuO3, BaRuO3, and solid solutions containing them.
(14) The method for manufacturing a substrate for an electronic device according to any one of the aforementioned (7) to (13), where the plasma is generated using laser light.
(15) An electronic device provided with the substrate for an electronic device according to any one of the aforementioned (1) to (6).
(16) The electronic device according to the aforementioned (15), which is a capacitor.
(17) The electronic device according to the aforementioned (15), which is a cantilever.